Gut passage times in two bivalve molluscs fed toxic microalgae: Alexandrium minutum, A. catenella and Pseudo-nitzschia calliantha

The occurrence of new phytoplankton species in a coastal area may be explained by the import of shellfish containing whole live algal cells in their digestive tracts. Indeed, shellfish containing toxic algal cells can induce both primary contaminations in safe areas (initially free from toxic microalgae), and secondary contaminations of other shellfish growing in the same area via the faeces of the imported animals. To mitigate this problem, shellfish need to be placed in a separate holding tank and their intestinal content purged. For a deeper understanding of the risks associated with transferring contaminated shellfish, oysters (Crassostrea gigas) and mussels (Mytilus edulis) were purposely fed either Alexandrium minutum or A. catenella (Dinophyceae) or Pseudo-nitzschia calliantha (Bacillariophyceae) toxic algae for 2 h. They were then transferred into individual tanks where they were continuously fed with a non-toxic alga, Tetraselmis suecica. Biodeposit production, faeces composition, and filtration rates were monitored for the shellfish over a 6-h period. The effect of temperature differences and different initial toxic algae concentrations were compared. This study revealed a relationship between temperature and cell lysis in the oyster digestive tract. It also indicated that toxic algae concentration did not seem to influence gut passage time in oysters, while a significant effect was observed in mussels, and confirmed the existence of a difference between oyster and mussel feeding patterns. L'apparition de nouvelles espèces phytoplanctoniques dans des zones côtières peut être expliquée par le transfert de coquillages contenant des cellules entières vivantes dans leur tractus digestif. En effet, des coquillages contenant des cellules algales toxiques peuvent entraîner une contamination primaire d'une zone exempte de ce phytoplancton, ainsi qu'une contamination secondaire via les fèces des autres coquillages cultivés dans cette zone. Pour pallier à ce problème, il faudrait mettre en bassin les coquillages afin de vider leur contenu intestinal. Pour une évaluation plus précise du risque lié au transfert de coquillages contaminés, des huîtres (Crassostrea gigas) et des moules (Mytilus edulis) sont placées en contact avec des algues toxiques (Alexandrium minutum, A. catenella et Pseudo-nitzchia calliantha), durant 2 heures, puis transférées dans des bacs individuels, dans lesquels ces bivalves sont nourris avec un apport en continu de Tetraselmis sueci